In recent years, nonaqueous electrolyte secondary batteries using metallic lithium, an alloy capable of storing and releasing lithium or a carbon material as the negative active material and a lithium transition metal complex oxide represented by the chemical formula: LiMO2 (M indicates a transition metal) as the positive active material have been noted as high-energy-density batteries.
A representing example of the lithium transition metal complex oxide is lithium cobaltate (LiCoO2), which has been already put to practical use as the positive active material for nonaqueous electrolyte secondary batteries.
However, other lithium transition metal complex oxides containing Mn or Ni as a transition metal, as well as those containing all of Mn, Ni and Co, have been also studied (for example, Patent Literatures 1 and 2 and Non-Patent literature 1).
Among those lithium transition metal complex oxides containing Mn, Ni and Co, the material having the same composition of Mn and Ni and represented by a chemical formula: LiMnxNixCo(1-2X)O2 is reported as showing a uniquely high thermal stability even in a charged state (high oxidation state) (for example, Non-Patent Literature 2).
It is also reported that the complex oxide having substantially the same composition of Ni and Mn has a voltage of approximately 4 V, as comparable to that of LiCoO2, and shows a high capacity and a superior charge-discharge efficiency (Patent Literature 3).
Batteries using a positive electrode containing, as a chief material (at least 50% by weight), such a lithium transition metal complex oxide containing Mn, Ni and Co and having a layered structure (for example, chemical formula: LiaMnbNibCo(1-2b)O2 (0≦a≦1.2, 0<b≦0.5)), because of their high thermal stability during charge, can be expected to achieve a marked reliability improvement. Also, it is reported (Non-Patent Literature 3) that this lithium transition metal complex oxide containing Mn, Ni and Co and having a layered structure, because of its high structural stability, exhibits better cycle characteristics than currently-used LiCoO2 or others, even when its charge voltage is set at a higher value (positive electrode potential of at least 4.5 V (vs. Li/Li+)) than values used in the current state of the art.
In existing nonaqueous electrolyte secondary batteries using a lithium transition metal complex oxide (for example, LiCoO2) for the positive electrode, an end-of-charge voltage is generally prescribed at 4.1-4.2 V. In this case, the positive electrode utilizes only 50-60% of its theoretical capacity. Therefore, the use of a lithium transition metal complex oxide having a layered structure enables the positive electrode to utilize at least 70% of its theoretical capacity and thus enables the battery to increase its capacity and energy density without marked deterioration of thermal stability even when a charge voltage is set at a high value.    Patent Literature 1: U.S. Pat. No. 2,561,556    Patent Literature 2: U.S. Pat. No. 3,244,314    Patent Literature 3: Patent Laying-Open No. 2002-42,813    Patent Literature 4: U.S. Pat. No. 2,855,877    Non-Patent Literature 1: Journal of Power Sources, 90(2000), 176-181    Non-Patent Literature 2: Electrochemical and Solid-State Letters, 4(12), A200-A200-A203 (2001)    Non-Patent Literature 3: Chemistry Letters, 2001, pp 642-643